十万个为什么·科学启蒙30篇（3）
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Why Do Onions Make You Cry?
为什么切洋葱会流泪
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Cutting an onion damages its cells and releases enzymes that convert sulfur compounds into volatile gases.
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One gas, syn-propanethial-S-oxide, rises into the air and dissolves in the moisture on your eyes’ surface.
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This forms mild sulfuric acid, which irritates the cornea and triggers tear glands to flush it out.
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Tears contain lysozyme—an enzyme that kills bacteria—so crying also protects your eyes from infection.
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Chilling onions before cutting slows enzyme activity and reduces gas release significantly.
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Using a sharp knife causes less cell damage than a dull one, lowering the amount of irritant produced.
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Wearing swim goggles or cutting near a running fan helps divert the gas away from your face.
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Onions bred for low sulfur content—like sweet Vidalias—produce far less of the tear-inducing compound.
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Garlic and leeks cause similar reactions, but their gases are less airborne and more pungent-smelling.
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Interestingly, some people lack the gene variant that makes them sensitive to this particular onion compound.